For more information on the client refer to the
engine-client repository.

What features does it have?

Maximum reliability. Connections are established even in the presence of:

proxies and load balancers.

personal firewall and antivirus software.

for more information refer to Goals and Architecture sections

Minimal client size aided by:

lazy loading of flash transports.

lack of redundant transports.

Scalable

load balancer friendly

Future proof

100% Node.JS core style

No API sugar (left for higher level projects)

Written in readable vanilla JavaScript

API

Server

Top-level

These are exposed by require('engine.io'):

Events

flush

Called when a socket buffer is being flushed.

Arguments

Socket: socket being flushed

Array: write buffer

drain

Called when a socket buffer is drained

Arguments

Socket: socket being flushed

Properties

protocol(Number): protocol revision number

Server: Server class constructor

Socket: Socket class constructor

Transport(Function): transport constructor

transports(Object): map of available transports

Methods

listen

Creates an http.Server which listens on the given port and attaches WS
to it. It returns 501 Not Implemented for regular http requests.

Parameters

Number: port to listen on.

Function: callback for listen.

ReturnsServer

attach

Captures upgrade requests for a http.Server. In other words, makes
a regular http.Server WebSocket-compatible.

Parameters

http.Server: server to attach to.

Object: optional, options object

Options

path (String): name of the path to capture (/engine.io).

policyFile (Boolean): whether to handle policy file requests (true)

destroyUpgrade (Boolean): destroy unhandled upgrade requests (true)

destroyUpgradeTimeout (Number): milliseconds after which unhandled requests are ended (1000)

See Server options below for additional options you can pass

ReturnsServer

Server

The main server/manager. Inherits from EventEmitter.

Events

connection

Fired when a new connection is established.

Arguments

Socket: a Socket object

Properties

Important: if you plan to use Engine.IO in a scalable way, please
keep in mind the properties below will only reflect the clients connected
to a single process.

clients(Object): hash of connected clients by id.

clientsCount(Number): number of connected clients.

Methods

constructor

Initializes the server

Parameters

Object: optional, options object

Options

pingTimeout (Number): how many ms without a pong packet to
consider the connection closed (60000)

pingInterval (Number): how many ms before sending a new ping
packet (25000)

maxHttpBufferSize (Number): how many bytes or characters a message
can be when polling, before closing the session (to avoid DoS). Default
value is 10E7.

allowRequest (Function): A function that receives a given handshake
or upgrade request as its first parameter, and can decide whether to
continue or not. The second argument is a function that needs to be
called with the decided information: fn(err, success), where
success is a boolean value where false means that the request is
rejected, and err is an error code.

Methods

send:

Sends a message, performing message = toString(arguments[0]) unless
sending binary data, which is sent as is.

Parameters

String | Buffer | ArrayBuffer | ArrayBufferView: a string or any object implementing toString(), with outgoing data, or a Buffer or ArrayBuffer with binary data. Also any ArrayBufferView can be sent as is.

Function: optional, a callback executed when the message gets flushed out by the transport

ReturnsSocket for chaining

close

Disconnects the client

ReturnsSocket for chaining

Client

Exposed in the eio global namespace (in the browser), or by
require('engine.io-client') (in Node.JS).

Support

Development

To contribute patches, run tests or benchmarks, make sure to clone the
repository:

git clone git://github.com/LearnBoost/engine.io.git

Then:

cd engine.io
npm install

Tests

Tests run with make test. It runs the server tests that are aided by
the usage of engine.io-client.

Make sure npm install is run first.

Goals

The main goal of Engine is ensuring the most reliable realtime communication.
Unlike the previous Socket.IO core, it always establishes a long-polling
connection first, then tries to upgrade to better transports that are "tested" on
the side.

During the lifetime of the Socket.IO projects, we've found countless drawbacks
to relying on HTML5 WebSocket or Flash Socket as the first connection
mechanisms.

Both are clearly the right way of establishing a bidirectional communication,
with HTML5 WebSocket being the way of the future. However, to answer most business
needs, alternative traditional HTTP 1.1 mechanisms are just as good as delivering
the same solution.

WebSocket/FlashSocket based connections have two fundamental benefits:

Better server performance

A: Load balancers
Load balancing a long polling connection poses a serious architectural nightmare
since requests can come from any number of open sockets by the user agent, but
they all need to be routed to the process and computer that owns the Engine
connection. This negatively impacts RAM and CPU usage.

B: Network traffic
WebSocket is designed around the premise that each message frame has to be
surrounded by the least amount of data. In HTTP 1.1 transports, each message
frame is surrounded by HTTP headers and chunked encoding frames. If you try to
send the message "Hello world" with xhr-polling, the message ultimately
becomes larger than if you were to send it with WebSocket.

C: Lightweight parser
As an effect of B, the server has to do a lot more work to parse the network
data and figure out the message when traditional HTTP requests are used
(as in long polling). This means that another advantage of WebSocket is
less server CPU usage.

Better user experience

Due to the reasons stated in point 1, the most important effect of being able
to establish a WebSocket connection is raw data transfer speed, which translates
in some cases in better user experience.

Having said this, attempting to establish a WebSocket connection directly so far has
proven problematic:

Proxies
Many corporate proxies block WebSocket traffic.

Personal firewall and antivirus software
As a result of our research, we've found that at least 3 personal security
applications block WebSocket traffic.

Cloud application platforms
Platforms like Heroku or No.de have had trouble keeping up with the fast-paced
nature of the evolution of the WebSocket protocol. Applications therefore end up
inevitably using long polling, but the seamless installation experience of
Socket.IO we strive for ("require() it and it just works") disappears.

Some of these problems have solutions. In the case of proxies and personal programs,
however, the solutions many times involve upgrading software. Experience has shown
that relying on client software upgrades to deliver a business solution is
fruitless: the very existence of this project has to do with a fragmented panorama
of user agent distribution, with clients connecting with latest versions of the most
modern user agents (Chrome, Firefox and Safari), but others with versions as low as
IE 5.5.

From the user perspective, an unsuccessful WebSocket connection can translate in
up to at least 10 seconds of waiting for the realtime application to begin
exchanging data. This perceptively hurts user experience.

To summarize, Engine focuses on reliability and user experience first, marginal
potential UX improvements and increased server performance second. Engine is the
result of all the lessons learned with WebSocket in the wild.

Architecture

The main premise of Engine, and the core of its existence, is the ability to
swap transports on the fly. A connection starts as xhr-polling, but it can
switch to WebSocket.

The central problem this poses is: how do we switch transports without losing
messages?

Engine only switches from polling to another transport in between polling
cycles. Since the server closes the connection after a certain timeout when
there's no activity, and the polling transport implementation buffers messages
in between connections, this ensures no message loss and optimal performance.

Another benefit of this design is that we workaround almost all the limitations
of Flash Socket, such as slow connection times, increased file size (we can
safely lazy load it without hurting user experience), etc.

FAQ

Can I use engine without Socket.IO ?

Absolutely. Although the recommended framework for building realtime applications
is Socket.IO, since it provides fundamental features for real-world applications
such as multiplexing, reconnection support, etc.

Engine is to Socket.IO what Connect is to Express. An essential piece for building
realtime frameworks, but something you probably won't be using for building
actual applications.

Does the server serve the client?

No. The main reason is that Engine is meant to be bundled with frameworks.
Socket.IO includes Engine, therefore serving two clients is not necessary. If
you use Socket.IO, including

<scriptsrc="/socket.io/socket.io.js">

has you covered.

Can I implement Engine in other languages?

Absolutely. The engine.io-protocol
repository contains the most up to date description of the specification
at all times, and the parser implementation in JavaScript.

License

(The MIT License)

Copyright (c) 2014 Guillermo Rauch <guillermo@learnboost.com>

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
'Software'), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.